March 4, 2014 | Scientists for several decades have studied the potential environmental impacts of a nuclear conflict—either an all-out conflagration between superpowers or a more limited regional war. Now a research team led by NCAR has produced an unusually detailed picture of the aftermath of a hypothetical regional nuclear war by using a modeling approach that includes simulations of atmospheric chemistry, the oceans, land surface, and sea ice.

The study, published this month in the American Geophysical Union journal Earth’s Future, finds that an exchange of 100 nuclear weapons between two regional adversaries would have more severe global implications for society and the environment than previously thought.

The research team’s model simulations show that global temperatures would drop initially by 1.5 degrees Celsius (about 2.7 degrees Fahrenheit) to their lowest levels in more than 1,000 years. The cooling would be caused by firestorms in major cities lofting ash and other particles high into the atmosphere, where they would block incoming solar heat. The colder temperatures would reduce precipitation, likely leading to widespread fires in regions such as the Amazon and pumping still more smoke into the atmosphere.

Whereas previous studies had projected that global temperatures would recover after about a decade, the new work indicates that cooling would persist at least 26 years, which is as far into the future as the simulations went. Two major factors would cause this prolonged cooling: an expansion of sea ice that would reflect more solar heat into space, and a significant cooling in the upper 100 meters (about 330 feet) of the oceans, which would warm only gradually.

The new study also tracked the influence of the urban firestorms on stratospheric chemistry. Approximately five teragrams of black carbon would be lofted up to the stratosphere, where it would spread globally. The smoke would absorb sunlight and heat the stratosphere, accelerating chemical reactions that destroy ozone. The resulting damage to the ozone layer would allow much greater amounts of ultraviolet radiation to reach Earth’s surface. The midlatitudes would experience a summertime UV increase of 30-80 percent.

The colder temperatures and higher UV levels could have widespread and potentially devastating impacts on society, the authors found. In addition to the destruction caused directly by the nuclear bombs, the colder temperatures worldwide would lead to killing frosts that would reduce growing seasons by 10-40 days per year for several years. The higher levels of UV would pose a threat to human health, agriculture, and terrestrial and aquatic ecosystems.

The research team used an NCAR-based computer model: the Community Earth System Model, which simulates interactive responses in atmosphere, ocean, land, and sea ice components of the Earth's climate system. For the atmospheric component, the team turned to the Whole Atmosphere Community Climate Model, which extends from the Earth's surface to the edge of space, and includes interactive calculations of stratospheric ozone chemistry and atmospheric dynamics.

The scientists ran a total of seven simulations, comparing a hypothetical war between two nations that have developed nuclear arms relatively recently (India and Pakistan) with control simulations in which there was no nuclear war.

“It’s such a complex process that you need sophisticated climate models to understand it,” said NCAR scientist Michael Mills, the lead author. “As we get a more detailed picture, we find that the atmospheric effects for a given amount of weapons deployed are even more severe than we previously thought.”

He added that the 100 relatively small nuclear bombs in the study represent just a small fraction of the world’s approximately 17,000 nuclear weapons.